In some imaging techniques, such as thermal imaging, radar, X-ray imaging, and ultrasound imaging, the acquired image is represented by a single-channel image. That is, an image carrying only intensity information. The image has thus only one channel, comprising intensity information, and may be presented as a grayscale image. However, various image processing techniques may be applied to the acquired image for enhancing different image parameters or for improving the presentation of the image.
For example, a histogram equalization may be applied on the single-channel image. Different histogram equalizations may have different purposes, for example enhancement of the image contrast or reduction of background noise in a particular part of the image. Many different types of histogram equalizations are known, such as plateau histogram equalizations, adaptive histogram equalizations, and high/low cut-off histogram equalizations. The paper “Review of Histogram Equalization Methods for Contrast Enhancement” (Chaudhari et al) discusses some of these histogram equalization methods. A known drawback with the use of histogram equalizations is that information in the single-channel image may be lost. For example, by applying a histogram equalization comprising a cut-off of a group of intensity values in a single-channel image, the information provided by the pixels having these intensity values may be lost.
Another example of an image processing technique is pseudo coloring, or palette which the technique also may be called. Each intensity value in the single-channel image is mapped to a color according to a table or function. The purpose of pseudo coloring is to present the single-channel image by colors instead of by grayscale, since it is easier for the human eye to identify a difference between different colors than between different shades of gray. Pseudo coloring is typically applied on the single-channel image at the end of the image processing process with a direct mapping of for example an 8-bit intensity data to an 8-bit color map, resulting in 255 different colors.
Different types of pseudo coloring are known. As an example, the paper “Frequency Domain Pseudo-color to Enhance Ultrasound Images” (Afruz et al) discloses pseudo-coloring based on frequency domain to visually enhance an image.
Even though much effort has been put to improve the image processing of single-channel images, there still exists a need for improvement within this field.